Generally, to protect a component against overvoltages, for example, during electrostatic discharges that may occur while the component is not connected, a protection component connected across the component to be protected is used. Such a protection component may, for example, be an avalanche diode, a bipolar transistor, or a unidirectional or bidirectional Shockley diode.
FIGS. 1A, 1B, and 1C show a portion of an optoelectronic circuit. A thin lightly-doped P-type silicon layer 1 (P−) is formed on an oxide layer 2, currently called BOX (“Buried Oxide”) layer. Layer 1, currently called SOI layer, comprises a portion entirely surrounded with silicon oxide, which forms core 5 of a waveguide. Core 5, being supported by oxide layer 2, is laterally delimited by two trenches 6 filled with oxide crossing silicon layer 1 and is covered with an upper oxide layer. In the illustrated example, the upper oxide layer comprises a central portion 7 surrounded with two lateral portions 9 deeper than the central portion 7. The central portion 7, which is shallower, results from a thermal oxidation and the lateral portions correspond to trenches filled with oxide, which do not cross the SOI layer 1.
In the following, a dimension of an element will be considered as being its width if this dimension is parallel to the width of the waveguide. The length of an element will be defined in the same way relative to the waveguide length.
FIGS. 1A and 1C show a portion of the waveguide where a germanium photodiode 10 is arranged along a portion of the waveguide length. Photodiode 10 is formed of a germanium block formed by epitaxy above a portion 11 of core 5. In the germanium block are formed, on the left-hand side of the drawings, a heavily-doped N-type cathode region 12 (N+) and, on the right-hand side of the drawings, a heavily-doped P-type anode region 14 (P+). Cathode 12 and anode 14 are separated by an intrinsic germanium or lightly-doped N- or P-type germanium region 15.
FIG. 2 is a curve 20 illustrating a current-voltage characteristic of an example of photodiode 10. Curve 20 characterizes the robustness of photodiode 10 against electrostatic discharges. Each point of curve 20 indicates current I (in Amperes) running through photodiode 10 for a 100-ns voltage step V (in volts). Up to an 11.5-V amplitude, current I increases slowly as a function of amplitude V of the voltage steps. As soon as applied voltage V exceeds 11.5 V, current I very strongly increases: this behavior corresponds to the breakdown of photodiode 10. A similar behavior can be observed with different values, whatever the specific structure of photodiode 10.